This invention relates to a biaspherical single lens for use in an optical head in an optical information recording-reproducing apparatus, and in particular relates to a very bright biaspherical single lens having NA of the order of 0.53.
In recent years, optical disks such as video disks, compact disks and magneto-optical disks as optical memories have been widely used as information recording carriers of large memory capacity.
To accurately record information highly densely on an information recording carrier of this kind and/or accurately reproduce recorded information therefrom, a resolving power of 1-2 .mu.m is required as the on-axis performance and the off-axis performance of the order of 0.5.degree. of an objective lens used in an optical information recording-reproducing apparatus. That is, an objective lens of NA of the order of 0.53 is necessary.
Also in the objective lens with such use, it is necessary to secure a sufficient spacing between the surface of a carrier such as an optical disk and the objective lens to thereby prevent the contact between the two and avoid damage to the recording carrier and the objective lens.
Further, in the above-described information recording-reproducing apparatus, the main current is a system in which the objective lens is moved in the direction of the optic axis or in a direction orthogonal to the direction of the optic axis to effect auto focusing or auto tracking. Consequently, in order to improve the response characteristic, compactness and light weight are required of the objective lens of this kind.
Heretofore, in Japanese Laid-Open Patent Application No. 58-42021, Japanese Laid-Open Patent Application No. 58-208719, Japanese Laid-Open Patent Application No. 60-122915, etc., a lens system comprising a four-group construction has been disclosed as the objective lens of this kind.
However, the objective lens shown in these laid-open publications is great in the full length of the lens system and cannot achieve the aforementioned compactness and light weight.
In order to eliminate the above-noted disadvantage, development of an aspherical single lens has recently been carried out actively, and the technique thereof is disclosed, for example, in Japanese Laid-Open Patent Application No. 57-201210, Japanese Laid-Open Patent Application No. 58-68711, Japanese Laid-Open Patent Application No. 59-23313, Japanese Laid-Open Patent Application No. 59-26714, Japanese Laid-Open Patent Application No. 61-2117, Japanese Laid-Open Patent Application No. 61-11715, Japanese Laid-Open Patent Application No. 61-56314, U.S. Pat. No. 4,027,952, U.S. Pat. No. 4,449,792, U.S. Pat. No. 4,571,034, U.S. Pat. No. 4,743,093, etc.
In the embodiments of the aspherical single lens shown in these publications, when the focal length of the lens is F, the thickness t of the transparent protective layer of a recording carrier to which the lenses shown in the embodiments are applicable is of the order of 0.26F-0.28F. However, to further improve the above-mentioned auto focusing and auto tracking operations, a lens of shorter focal length is desired in respect of compactness and light weight. Further, considering on the basis of the same NA, making the focal length of the lens shorter is equivalent to making the effective light beam diameter smaller, and it is possible to make other optical parts such as prisms in the optical head used in the optical information recording-reproducing apparatus smaller in size and as a result, it becomes possible to make the entire optical head light in weight, compact and thin. Accordingly, this is desirable for the higher speed of the movement of the optical head in the radial direction of the optical disk, i.e., the seek operation.
It is an object of the present invention to eliminate the above-noted disadvantages peculiar to the prior art and to provide a biaspherical single lens for an optical information recording-reproducing apparatus which is as bright as NA of the order of 0.53 and yet has a good imaging performance both on the axis and off the axis.
The above object of the present invention is achieved by the biaspherical single lens of the present invention which will hereinafter be described.
The biaspherical single lens of the present invention for forming an image through a substrate of thickness t is a biaspherical single lens having its first and second surfaces both formed by an aspherical surface and in which when the distance from any point on the aspherical surface to the tangential plane of the vertex of the aspherical surface is X and the distance from said any point to the optic axis is H and the reference radius of curvature of the .nu.th surface (the radius of curvature of the vertex) is R.nu. and the cone constant of the .nu.th surface is K.nu. and the aspherical coefficient of the .nu.th surface is A.nu.i (i=3, 4, ...), the aspherical surface is an aspherical surface expressed by the following equation and satisfies the following conditions (1), (2), (3) and (4): ##EQU2## where F is the focal length of the aspherical single lens, t is the thickness of the substrate, D is the thickness on the optic axis of the aspherical lens, and N is the refractive index of the aspherical single lens for the wavelength used.